Water jet propulsion units are of two main kinds, a mixed flow and an axial flow configuration. A mixed flow unit is one in which the water enters the impeller parallel to the shaft and is directed radially from the shaft and leaves the impeller with radial and axial velocity. An axial flow unit is one where the water enters the impeller parallel to the shaft and also leaves the impeller parallel to the shaft. The differences are more fully explained in the publication "Jet Boating", November 1986, Volume 6, No. 8, page 46.
An example of an axial flow unit may be seen in New Zealand Patent Specification 123,228 where there is described a motor with two impellers, that is a two stage motor having a set of stators between the two impellers and another set of stators in the rear nozzle of the jet unit.
In DE 3942672 A1 there is described a mixed flow water jet propulsion unit. In the embodiments described there are two or three impellers in the pump section whose casing diverges from a narrower cross-sectional area at the inlet to a maximum cross-sectional area at the middle and converges to the minimum cross-sectional area at the outlet. The impellers are counter-rotating with respect to each other.
The provision of counter-rotating propellers mounted on concentric shafts is well known from the prior art, for example in U.S. Pat. Nos. 4,642,059; 4,832,570; 5,030,149; 5,087,230 and in WO 93/01085. It is desirable to use a concentric configuration in jet propulsion units so as to minimize obstructions causing turbulent flow within the pump casing and also to achieve maximum reliability under the extreme conditions encountered in a water jet propulsion unit.
It is an object of this invention to go some way towards achieving these desiderata or at least to offer the public a useful choice.